Signal correction circuits



G. C. SZIKLAI ETAL BY @FOM TTORNEY @rates This invention relates to thereinsertion of direct current (DC.) and/or low frequency components ofelectrical signals, and particularly, but not exclusively, to thereinsertion of such components in a television system. This applicationis a continuation of application Serial No. 312,598, led October l,1952, now abandoned.

Various circuits have been proposed for reinsertion of theabove-mentioned components where the components have been suppressedduring transmission, as, for example, where they are suppressed bytransformers or by alternating current amplifiers. These circuits dependfor their operation upon the transmission of periodically recurringcontrol pulses which are caused to go to a iixed voltage level, such asblack, in a picture, or a few volts beyond black before the saidcomponents have been suppressed. In television systems, these recurringpulses usually are the blankinfy and synchronizing pulses. An excellentdiscussion of the problem of DC. insertion with analysis of varioussuggested forms of reinserting circuits is contained in an articleentitled Television D.C. Component by Karl R. Wendt, commencing on page85 of the March 1948 issue of the RCA Review.

Among the various proposed circuits are the so-called keyed or clampcircuits of which the circuit described in U.S. Patent No. 2,299,945 toWendt is an example. In general, keyed circuits have advantages overother forms of D.C. restorers in that they can be very fast operatingwith low distortion and high noise immunity, and can operatesatisfactorily with signal levels much lower than those required forother forms of restorers.

in the double keyed diode type of clamp circuit as described in theaforesaid patent to Wendt, the charge of the coupling capacitor in thegrid circuit of the tube which is to have the reinserted DC. componentin its output is controlled by means of two diodes which are keyed byperiodically recurring pulses, such as synchronizing pulses. Upon theoccurrence of a synchronizing or keying pulse, one of the diodes isrendered conducting and the capacitor will discharge therethrough acertain amount if its charge should be reduced to provide the correctDC. reinsertion, or it will charge a certain amount through the otherdiode if its charge should be increased. Between keying pulses, bothdiodes .are held non-conducting by means of a biasing voltage.

Another known type of clamp circuit which is also bidirectional (i.e.the charge on the coupling capacitor may be increased or decreasedduring the clamp interval) employs two keyed triodes instead of twokeyed diodes. This keyed double triode type of reinserting circuit isdescribed by the Radar School Stati on pages 2-35 et seq. of Principlesof Radar, published by McGraw-Hill Book Company, 1946 edition. Anothersimpler forrn of clamp circuit is the single keyed diode or unbalancedclamp circuit which is described in U.S. Patent No. 2,299,944 of Wendt.its operation is somewhat similar to the double keyed diode circuit butit employs only a single keyed diode, utilizing the gridcathode path ofthe tube being clamped as the additional diode.

While these three known types of clamp circuits mutually possess thegeneral advantages of keyed circuits such as capability for high speedoperation with low distortion and high noise immunity, and low signallevel requirements, there are various advantages and disadvantagespeculiar to each form which differentiate the desirability andapplicability of each. The double keyed diode circuit can be balanced sothat keying pulses will not appear on the signal; also the clampinglevel can be easily adjusted, and the circuit can be used with equalease to clamp on positive or negative peaks of the signal. However,disadvantages to the use of the double keyed diode circuit include thecomplication of providing synchronous keying pulses of oppositepolarities to effect push-pull keying of the two diodes, and therequirement that the keying pulses must be of greater amplitude than thesignal in order that a bias may be used that will insure that neitherdiode conducts between keying pulses.

The keyed double triode circuit requires keying pulses of only onepolarity, and the keying pulses need not be as large as the signal ifnegative peaks of the signal are clamped. However, since the keyeddouble triode circuit is not balanced some keying pulse component willappear on the signal; Aalso the clamping level cannot be adjustedeasily. While the single keyed diode circuit is the simplest of thethree types, the circuit can be used only for clamping on positive peaksof the signal; the clamping level is not adjustable but fixed at thecathode potential of the tube being clamped; and the circuit iSinherently unbalanced with the result that some of the pulse will beapplied to the signal.

The present invention is an tion circuit of the clamp type whichcombines many of the advantages of the known types of clamp circuits asset forth above. Relying upon the bidirectional switch properties of ajunction transistor, as revealed in the copending application of GeorgeC. Sziklai, Serial No. 308,618, tiled on September 9, 1952, now PatentNo. 2,728,857, granted December 27, 1955, and entitled ElectronicSwitching, this 'invention employs a single junction transistor as abidirectional switch in the grid circuit of the tube to be clamped.Periodically, keying pulses applied to the base electrode of thejunction transistor render the emitter-collector path of the transistorconductive for a clamping interval. Since current may then iiowtherethrough in either direction, as discussed in the aforesaidapplication, thev single transistor emittercollector path servesalternatively as a charging or disimproved signal correcr charging pathfor the coupling capacitor in the grid circuit ofthe tube to be clamped.

It will be seen that while the present invention provides a D.C.restorer capable of fast, essentially distor- 'tionless, noise-freeoperation over a Wide range of signal levels, it is exceptionally simplein form. The clamp circuit of the present invention requires keyingpulses of only one polarity. The circuit may have the advantage ofbalanced arrangements in that a keying pulse component will not appearon the clamped signal. The clamping level can be easily adjusted over asubstantial range, and the circuit may be used to clamp on eitherpositive or negative peaks ofthe signal.-

It is an object or the present invention to provide improved directcurrent and/ or low frequency components reinserting or correctingapparatus and, especially, improved and simplified reinserting apparatusof the keyed type. v

`A further object of the present invention is to provide an improved andsimplified signal correction circuit o f the type in which a condenserin the circuit is keyed for both charge and discharge.

An additional object of the present invention is to provide a televisionsystem with an improved and simplied D.C. reinsertion circuit of theclamp type.

Another object of the present invention is to provide a televisionreceiver with av simple clamp circuitfor restoring D.C. and/or lowfrequency components to a video signal.

A further object of the invention is to provide a simple clamp `circuitemploying a junction transistor as a bidirectional switch.

An additional object of the invention is to provide a simple D.C.restorer of the keyed type which requires keying pulses of only onepolarity.

Another object of the invention is to provide a D.C. reinsertioncircuit, capable of high speed operation with low distortion and highnoise immunity, which is simple, accurate and stable.

Other and incidental objects and advantages of the invention will beapparent to those skilled in the art from a reading of the followingspecification and an inspection of the accompanying drawings in which:

FIG. l is a schematic circuit diagram of a signal correction circuit,illustrating an embodiment of the present invention employing a p-n-pjunction transistor.

FIG. 2 is a schematic circuit diagram showing the ernbodimentillustrated in FIG. l as applied to use in a television receiver.

lFIG. 3 is a schematic circuit diagram illustrating another embodimentof the present invention employing an n-p-n junction transistor andapplicable to Iuse in the television receiver shown in FlG. 3. l

In FIG. l a source 11 of video signals is coupled by a capacitor 13 toan input terminal or electrode 17 of a subsequent stage or utilizationdevice in a video signalling system. The source 11 may be some stage ofa television transmitter or receiver, for example, the video signaloutput of which has lost or erroneously represents its D.C. and/ or lowfrequency components. The stage incorporating input electrode 17 maythen be, for example, some subsequent utilization device or operatorupon the video signal, such as the modulating ampliiier in a transmitteror the image reproducing device in a receiver, for which it is desirableor requisite that the video signal input thereto contain the proper D.C.and low frequency components. The input electrode 17 may be the controlgrid of an electron tube 15, as shown, or may, for example, lbe theinput electrode of a transistor.

The video signal wave 12 appearing as the output of.

source 11 has periodically recurring control periods x, such as theblanking intervals in a television signal, during which occur periodicreference and control signals, such `as blanking and synchronizingpulses. As in other forms of keyed resto-rers a source 41 suppliesperiodically recurring keying pulses 42 to key into operation, during aselected portion of each control period, a circuit for adjusting thecharge on capacitor 13 whereby the grid 17 of tube 15 may be brought tothe same predetermined potential (i.e. the clamping level) during eachcontrol period. The keying pulses 42 may be derived in source 41 fromsynchronizing pulses separated from the video signal .12, or may bederived from ilyback pulses occurring in detection wave generato-rsdriven by such separated synchronizing pulses, or may be derived in someother manner from other apparatus associated with the particular videosignalling system in which the invention is ernployed. Whatever thenature of the derivation of the keying pulses 42, it is essential thatthe keying pulses 42 occur synchronously with some flat, recurrent,reference level portion of the Video signal, such as the synchronizingpulse peaks or the back porch portions of the blanking pedestals.

The charge adjusting circuit of the present invention is unique in thata single electrical path serves alternatively as a charging anddischarging path for the capacitor 13. This is made possible byutilization of the bidirectional current characteristics of a junctiontransistor, as revealed in the aforesaid copending Sziklai application.

A junction transistor 2li of the p-n-p type is shown in FIG. 1. Thetransistor comprises a body of semiconductive material, such asgermanium or silicon, having two p-type regions 21 and 2S, separated byand contiguous with opposite surfaces of an n-type region 23. Electricalbarriers, as discussed in U.S. Patent No. 2,569,347 to William Shockley,issued on September 25, 1951, occur at the interfacial junctions 27, 29.The electrodes 31, 33 and 35, by which external circuit connections aremade to the respective regions '21, 23, 25, make essentially ohmic(non-rectifying) contacts with their respective regions. Thepractitioner of the present invention who should desire a theoreticalbackground on junction transistors in general may refer to the aforesaidShockley patent and to the following publications for a preliminaryknowledge of the nature of the junction transistor, some of its betterknown characteristics, and projected theories of its operation: TheTheory of p-n Junctions in Semiconductors and p-n Junction Transistors"by W. Shockley, appearing in Volume 23 (1949) of the Bell SystemTechnical Journal, starting at page 435; Electrons and Holes inSemiconductors by W. Shockley, published by D. Van Nostrand Co. in 1950;p-n Junction Transistors by W. Shockley, M. Sparks, and G. K. Teal,appearing in Volume 83 of the Physical Review, starting at page 151 inthe July 1, 1951, issue; Some Circuit Properties and Applications ofn-p-n Transistors by R. L. Wallace, Jr. and W. J. Pietenpol, appearingin Volume 39 of the Proceedings of the LRE., starting at page 753 in theJuly 1951 issue. Y

In accordance with conventional nomenclature in the transistor eld, theelectrodes 31, 33, and 35 will be referred to as emitter, base, andcollector, respectively. However, it will be appreciated, particularlyin view of the bidirectional character of the current flow through theemitter-collector path of the transistor 20, that the designation ofelectrode 31 as emitter and electrode 35 as collector is essentiallyarbitrary yand not intended to be restrictively indicative of theirrespective functions.

In the circuit shown in FIG. 1 the collector electrode 35 is connecteddirectly to the grid 17 of the tube 15, the cathode 19 thereof beingconnected to ground. The emitter electrode 31 of the transistor 2i) isadjustably connected to` a point of reference potential, negativerelative to ground, such as via an adjustable tap to a point on avoltage dividing resistor 47 connected between ground and a source ofnegative potential. The base electrode 33 is returned to ground via aresistor 45. There is thus established a bias in the reverse directionbetween base 33 and emitter 31, which renders the emitter-collector pathof the transistor 20 normally non-conductive.

However, whenever a keying pulse 42, of negative potential relative toground, is applied from source 41 via capacitor 43 to the -baseelectrode 33, the reverse bias is overcome and a bias in the forwarddirection is ternporarily established between ba-se 33 and emitter 31which renders the emitter-collector path of the transistor 20 conductivefor the duration of the keying pulse 42. A by-pass capacitor 46 may beprovided between the emitter 31 and ground to avoid degenerativecoupling between the base-emitter and emitter-collector circuits, and torender the clamping operation substantially insensitive to variations inkeying pulse amplitude. AAs has been stated before, the keying pulses 42are synchronously timed to coincide lwith some iiat recurrent portionsof the signal Wave 12 V(eg. portions of the control signals which occurduring control period .r, such as synchronizing pulse peaks or backporch portions of blanking pedestals if signal 12 is a conventionalcomposite television signal). While in many television applications ofthe present invention it will be preferable to key during back porchblanking intervals, for purposes of explanation it' will be assumed thatthe keying pulses 42 are timed to occur during the ultimate pulse peaksof the video signa-l wave 1'2. Further, since it is prudent in avoidingerroneous level setting to provide that the keying pulse will not lastbeyond the selected signal reference level, it will be assumed that thekeying pulses 42 are timed to coincide with the portions "y of theultimate pulse peaks occurring during control periods x of the signalwave 12.

Thus, during each recurrent interval y of the signal 12, theemitter-collector path of the transistor 26 is rendered conductive bythe action of a keying pulse 42, and current will ilow therethrough ineither direction, the direction of ow depending upon the polarity of thepotential difference between the signal level at grid 17 and thereference potential or clamping level voltage (which is determined bythe tap selection on divider 47 of the emitter electrode potential). lfthe D.C. component were present in the signal output of source 11, thesignal 12 should appear at the grid 17 at the same level during eachreference interval y. But, when the signal has suiered the loss of itsDC. component as under the conditions assumed here, the signal level atgrid 17 during reference intervals y varies with the degree of signalswing between reference intervals. However, by adjusting the charge onthe coupling capacitor 13 the necessary amount to bring the grid 17 tothe selected clamping level during each reference interval, and byholding the charge at its adjusted value until the subsequent referenceinterval, the signal output of tube 15 includes the proper DC.component.

Thus, if the signal level at grid 17 at the beginning of a reference orclamping interval y is greater in magnitude than the clamping levelvoltage, then the capacitor 13 will discharge through theemitter-collector path of transistor 2t; during the clamping intervaluntil the potential of grid 17 is brought equal to the clamping level;but if the signal level at grid 17 at the beginning of a clampinginterval y is smaller in magnitude than the clamping level voltage, thena charging current will ilow in the opposite direction through theemitter-collector path during the clamping Vinterval until the potentialof grid 17 is brought equal to the clamping level. As the keying pulseends, the base-emitter circuit of the transistor 241 returns to areverse bias condition and the emittercollector path returns to anon-conductive state, leaving no path for current ow to change thecharge on capacitor 13. As the content of the signal portionsintervening the reference intervals varies, the charge of the capacitor13 is adjusted the proper amounts and in the appropriate directions bythe keyed circuit to continually return the grid 17 to the clampinglevel each reference interval.

lt may be appreciated that it would be desirable, though not essential,that the junction transistor 2l) employed in the present invention besymmetrical in the sense discussed in the aforementioned copendingapplication of Sziklai: i.e. that current characteristics for bothdirections of current how through the emitter-collector path beessentially symmetrical. Not all junction transistors attain thiscondition of symmetry; primarily as a consequence of the particularprocedure employed in their fabrication or development, some junctiontransistors present a substantially greater impedance to current ow yinone direction between the outer zones, for a given set of biasconditions, than they present to current llow in the opposite directionbetween the outer zones under equivalent bias conditions.

While there are many contributing factors which may ldetermine thepresence or lack of such symmetry in a junction transistor, it isbelieved by the applicants that if the resistivities of the two outerzones are substantially equal and if the two junctions are symmetrical(ie. if the junction between one outer zone and the intermediate zone issubstantially equal in magnitude or extent to the junction between theother outer zone and the intermediate zone) a sufficient degree ofsymmetry in current characteristics is achieved to permit considerationof the unit as a symmetrical junction transistor.

Thus it should be stated that, though successful clamping action may beachieved with the present invention using asymmetrical transistor units,the circuit is operated to full advantage with maximum accuracy if thetransistor 2G is a substantially symmetrical unit.

A particular application of the present invention to use in a'television receiving system is shown in FIG. 2. A conventionaltelevision signal receiver 51 is provided for receiving and demodulatinga transmitted television carrier wave. Briey, it may comprise carrierwave amplifying apparatus, a frequency converter, and a signal detectorby means of which composite television signals, including video andcontrol signals, such as synchronizing and blanking pulses, arerecovered from the carrier wave. A conventional video amplier 53,coupled in the usual manner to signal receiver 51, amplies the compositesignal output of the receiver.

The video amplifier 53 is capacitively coupled by condenser 13 to theinput electrode 57 of an image reproducing device 55, which may be akinescope of conventional type, having the customary components such asa deflection yoke 69 and an electron gun including a cathode S9 andcontrol grid 57. Also coupled to the video amplitier 53 is the syncsignal separator 61, which functions in the usual way to separate thehorizontal and vertical synchronizing pulses from the vdeo signals andalso from one another.

The separated horizontal and vertical synchronizing pulses are impressedrespectively upon horizontal dellection generator 63 and verticaldecction Igenerator 67. These generators also function in the usualmanner to produce respectively sawtooth wave energy at horizontal andvertical deflection frequencies for control, via deection yoke 69, ofthe electron beam dellection in the image reproducing device 55.

As in the previously described diagram, a junction transistor 20 isprovided, having emitter, base and collector electrodes, 31, 33, and 35respectively. The emitter 31 is adjustably connected to a point ofnegative potential as by means of agtap on voltage divider 47, thelatter being connected between ground and a source of negativepotential. A by-pass capacitor 46 may be connected between the emitter31 and ground. The collector 35 is connected to grid 57, the beamintensity control electrode of the image reproducing device 55. Cathode59, the electron emitter of image reproducing device 55, is connected toground. The base electrode 33 of the transistor 20 is also returned toground via resistor 45.

With the connections of the transistor electrodes as shown, there isestablished a bias in the reverse direction between base 33 and emitter31, which render the emittercollector path of the transistor 20 normallynonconductive. However, when a negative keying pulse 62, appearing atterminal H of the horizontal deflection generator 63, is applied viacapacitor 43 to the base electrode, the reverse bias is overcome and abias in the forward direction is temporarily established between base 33and emitter 31 which renders the emitter-collector path of thetransistor 20 conductive bidirectionally for the duration of the keyingpulse 42.

n the system shown in FIG. 2 the periodic keying pulses 62 may bederived from the llyback pulses which normally occur in the operation ofconventional deflection generators, such as the horizontal deflectiongenerator 63. Time delay and wave shaping apparatus may be required inoperation on the ilyback pulses to suitably time the derived keyingpulses 62 to occur during the desired portions of the television signalscontrol periods. As a practical example, each keying pulse 62 may betimed to occur during a selected interval of the back porch blankingpedestal 54h,` of the television signal 54, appearing in the output ofthe video amplifier 53. If

this is the selected mode of operation, then periodically as a backporchblanking portion 54C of the'sign'al 54 appears at grid 57 of thereproducing device 55, a keying pulse 62 synchronously occurs to key theemittercollector path of transistor 20 into a conductive state. If thereis any potential difference between the signal level at grid 57 and theselected clampinglevel voltage (determined by the tap selection of thepotential of emitter 31) at this time, the capacitor 13 will be subjectto either aA charging or a discharging action via the emitterrdeflection generator collector path of transistor Ztl, whichever isrequired `to bring the grid 57 to the clamping level voltage. Thus thebrightnessof picture elements will oe properly represented in the imagereproduced by device 55, despite variations in background illuminationof the original scene, due to the action of the keyed correction circuitin adjusting the charge of coupling capacitor 13 to continually bringthe signal level for back porch blanking pedestals 54C (representativeof picture black) to a. xed clamping level, and holding the charge ofthe capacitor 13 at the adjusted value throughout each subsequent videosignal interval.

The marked advantages of the present invention as a clamp circuit forrestoring to a video signal the DC. component, which it has lost whilepassing through apparatus such as a conventional A.C. coupled videoampliiier, are readily apparent. Fast, essentially distortionless,noise-immune operation is achieved with a very simple circuit, requiringa single junction transistor; keying pulses of only one polarity arerequired. The clamping level is easily adjusted, as by changing thepotential of the emitter electrode 31 in the illustrated embodiment. Thecircuit may be used with signals of eitherpolarity (eg. the circuit maybe used for clamping in television stages where the sync and blankingpulses in the signal input extend in the positive direction as well asin stages, such Vas that shown in FIG. 2, where thesync and blankingpulses in the signal input extend in the negative direction). Also,particularly where a symmetrical transistoris used, as discussedpreviously, the clamped signal will be essentially free of any keyingpulse cornponent.

FIG. 3 has been included in the drawings to illustrate that junctiontransistors of the n-p-n type may be ernployed in the present inventionas well as junction transistors of the p-n-p type. A junction transistor70 is illustrated, having two n-type zones 71 and 7S, separated by andcontiguous with opposite surfaces of a p-type zone 73, the inter-zonejunctions being designated as-77 and 79, respectively. The emitter, baseand collector electrodes, 81, 83, and 85, respectively, by whichexternal circuit connections are made to the respective regions '71, 73,and 7S, make essentially ohmic (non-rectifying) contacts with theirrespective regions.

In substituting the n-p-n circuit of FIG. 3 for the keyed p-n-p circuitin FIG. 2, the collector 85 is connected to the grid 57 of the imagereproducing device 55. The emitter 81 is adjustably connected to a pointof negative potential, as by a tap on the voltage divider 47, which isconnected between -ground and a source of negative potential. As in thepreviously described diagrams, a by-pass capacitor 46 may be connectedbetween the emitter 81 and ground. A bias in the reverse directionbetween base 83 and emitter 81 is established by returning the base 83via resistor 45 to the negative end of the voltage divider 47. Toperiodically overcome this reverse bias and thus render theemitter-collector path temporarily conductive, the keying pulses 62A,which are applied from terminal H to the base 83 via the capacitor 43,must be of positive polarity, as contrasted with the negative pulsesrequired in the p-n-p circuit. The keying pulses 62A also may be derivedfrom the periodic ilyback pulses occurring in the operationl of thehorizontal 63 and timed to occur during the desired reference levelportions of the signal 54.

Operation of the n-p-n circuit in the receiving system of FIG. 2 issimilar to the previously described operation of the p-n-p circuit.Thus, when each blanking pedestal 54C appears at grid 57, a keying pulse62A keys the emitter-collector path transistor 70 into its(bidirectionally) conductive stage. 1f the signal'level at grid 57differs from the clamping level voltage (determined by the selectedpotential of emitter Si), current will flow through theemitter-collector path of transistor 70 in the appropriate direction toadjust the charge on capacitor 13 to bring the grid 57 to the clampinglevel. When keying pulse 62A ends, the base-emitter circuit oftransistor 70 returns to a reverse bias condition, and theemitter-collector path of transistor '70 returns to a nonconductivestate, leaving no path for current ow to change the charge on capacitor13 until the occurrence of the next keying pulse.

What is claimed:

l. In a circuit for correction of a signal supplied from a source andhaving recurring control periods, the combination including an elementto which it is desired to apply a corrected signal, a capacitorconnected in series with said source and said element, a junctiontransistor having base, emitter and collector electrodes, a charging anddischarging circuit for said capacitor, said circuit being connectedbetween said element and a point of reference potential and includingthe emitter-collector path of said junction transistor, biasing meansconnected between said base electrode and said point of referencepotential for biasing said base electrode in the reverse direction suchas to render the emitter-collector path of said junction transistornormally nonconductive, and keying means coupled to said base electrodefor rendering said emitter-collector path conductive during a portion ofeach of said control periods, said keying means applying a keying pulseto said base electrode to overcome said reverse bias during each of saidcontrol period portions, said ernitter-collector path completing acharging circuit for said capacitor when the potential diierence betweenthe potential of said element and said reference potential is of onepolarity during a control period portion, and said emitter-collectorpath completing a discharging circuit for said capacitor when saidpotential difference is of the opposite polarity to said one polarityduring a control period portion.

2. In a television system including a source of video signals the directcurrent component of which has been at least partially removed, anelement to which it is desired to apply corrected video signals, and acapacitor coupling said source to said element, the combinationincluding a switching device comprising a body of semiconductivematerial having two zones of one conductivity type and a third zone ofthe opposite conductivity type between and in contact with said twozones, means for applying a bias in the reverse direction between saidthird zoneand one of said two zones, means for connecting said one ofsaid two zones to a point of reference potential, means or connectingthe other of said two zones to said element, and means coupled to saidthird zone to apply thereto keying pulses for periodically overcomingthe bias in the reverse direction whereby current may ilow between saidtwo zones of said switching device in a iirst direction when the signalpotential at said element exceeds said referencepotential and in adirection opposite to said rst direction when said reference potentialexceeds the signal potential at said element.

3. A television system in accordance with claim 2 wherein said videosignals have recurring control periods and wherein said keying pulsesare applied to said third zone only during a portion of each of saidcontrol periods.

4. In a television receiver including a video amplier, an electrondischarge device having a control electrode, and a capacitor couplingsaid video ampliiier to said control electrode, a clamp circuitincluding a switching device comprising a body of semiconductivematerial having two zones of one conductivity type and a third zone ofthe opposite conductivity type between and in contact with said twozones, means for applying a bias in the reverse direction between saidthird zone and one of said two zones, means for connecting said one ofsaid two zones to a point of reference potential, means for connectingthe other of said two zones to said control electrode, a source ofkeyingpulses, and means for coupling said source to said third zone, each ofsaid keying pulses being of such polarity and amplitude as to overcomesaid reverse bias and permit current to flow between said two zones in adirection determined by the magnitude of the signal potential on saidcontrol electrode relative to said reference potential whereby saidcontrol electrode periodically is brought to said reference potential.

5. A television receiver in accordance with claim 4 wherein the videosignal output of said video amplifier has recurring blanking intervalsand -Wherein said keying pulses occur only during portions of saidblanking intervals.

6. In a video signal translating system wherein a capacitor couples anoutput terminal of one stage of said system to an input terminal of asubsequent stage thereof, a keyed clamp circuit comprising a junctiontransistor having base, emitter and collector electrodes, means forconnecting one of said emitter and collector electrodes to said inputterminal, means for connecting the other of said emitter and collectorelectrodes to a point of reference potential, means for biasing saidbase electrode in the reverse direction so as to render theemitter-collector path of said junction transistor normallynonconducting,

electrode, the direction of current flow in said emittercollector pathduring each ofthe periods of conduction eing dependent upon the polarityof potential difference between the signal potential at said inputterminal and said reference potential during said period of conduction.

References Cited in the ile of this patent UNITED STATES PATENTS SziklaiDec. 27, 1955 Trousdale Feb. 11, 1958

